The goal of this project is to continue to enable the investigation of the electrostatic properties of macromolecules for biomedical research by supporting the maintenance and continued development of the open-source Adaptive Poisson-Boltzmann Solver (APBS) software. Among the various inter- and intermolecular interactions in biomolecules, electrostatic energetics are of special importance due to their long range and the substantial charges of biopolymer components. APBS is a free, open-source software package designed to solve the equations of continuum electrostatics for large biomolecular assemblages using new highly-scalable parallel algorithms. This software was designed "from the ground up" using modern software design methods to facilitate its modification and integration with other software packages. In order to ensure the continuing availability of APBS, we propose to ensure its suitability for both end-users and software developers in the biology community through continuing maintenance; to improve existing features based on new algorithms and user feedback; to incorporate new features based on the needs of the research community; and to expand the APBS user base by educating users, improving the efficiency and portability of the code, and facilitating interaction with other biomolecular software simulation packages.